Commonly referred to as GRASS, this is free Geographic Information System (GIS) software used for geospatial data management and analysis, image processing, graphics/maps production, spatial modeling, and visualization. GRASS is currently used in academic and commercial settings around the world, as well as by many governmental agencies and environmental consulting companies. GRASS is an official project of the Open Source Geospatial Foundation.
Grass – image processing modules
“Satellite imagery and orthophotos (aerial photographs) are handled in GRASS as raster maps and specialized tasks are performed using the imagery (i.*) modules. All general operations are handled by the raster modules.imageryintro: A short introduction to image processing in GRASS 6.
GRASS – image classification modules
Information about the image classification process and modules in GRASS can be obtained from: http://grass.osgeo.org/wiki/Image_classification
GRASS – FAQ
The GRASS FAQ/ Wiki links to plenty of GRASS specific howtos which makes the learning curve of GRASS easier and provides fast help for many tasks.